This paper provides a unified semantic analysis of the so-called &#8216;out-of-control&#8217; circumfix <i>ka-&#8230;-a</i> in St&#8217;&#225;t&#8217;imcets (Lillooet Salish). <i>ka-&#8230;-a</i> expresses an initially puzzling range of meanings, including &#8220;be able to&#8221;, &#8220;manage to&#8221;, &#8220;suddenly&#8221;, &#8220;accidentally&#8221;, and &#8220;non-controllable&#8221;. We propose that <i>ka-&#8230;-a</i> encodes circumstantial modality; we show that its various meanings all reduce to either an existential (ability) or universal (involuntary action) interpretation. Our analysis provides further support for a striking difference between St&#8217;&#225;t&#8217;imcets and English. In English, modals lexically encode quantificational strength, but do not encode distinctions between epistemic, deontic and circumstantial interpretations. St&#8217;&#225;t&#8217;imcets modals display exactly the inverse pattern (Rullmann et al. 2008). In line with this, <i>ka-&#8230;-a</i> lexically encodes circumstantial modality, but does not encode quantificational strength. The parallel between <i>ka-&#8230;-a</i> and other St&#8217;&#225;t&#8217;imcets modal elements provides support for our analysis, in contrast to previous accounts (e.g., Demirdache 1997), which treat <i>ka-&#8230;-a</i> as primarily aspectual in nature.